Fiber optic rotary joint

A Fibre optic Rotary Joint is equivalent to an electrical slip ring but in the domain of optics and is mostly used as a co-existing element along with slip rings. Fibre optic Rotary Joint (FORJs) are available in single and multi-channel forms and have a special characteristic of having unlimited reach.

A Fibre Optic Rotary Joint (FORJ) is a device allowing a light or optic signal to be transferred across the interface between a stationary support structure and its continuously rotating platform. The FORJ applications have multiplied with the increasing adoption of fiber optic joints for communication transmission.

FORJ’s are used in a plethora of applications like radar pedestals, wind turbines, as well as electro-optic sensors, have implemented fiber optic rotary joints to take care of optical signals simultaneously with slip rings to handle electrical power and signals.

FORJs can be divided into categories basis mode of operation:
• Single or multi-pass
• Passive or active.

Passive FORJs is responsible for transferring an optical signal between the rotating and stationary structure without the support of electronic processing with the help of elements like filters and lenses that can be used to interpret the optical signal.

Active FORJs uses the principle of electronics to process the signal to positively influence rotor to stator transmission properties and involves electrical/ optical conversions, amplification, etc.

An example of the implementation of the active FORJ can be the medical CT scanner which can be used to carry high-speed image data from the rotating X-ray detectors to the stationary data processors.

Process of working of FORJs

Single Pass FORJ

Single-pass FORJs are rotary joints where a single fiber enters the FORJ from one of the sides of the rotating assemblage making the optical signals combined between them as one fiber rotates relative to the other. It follows the basic principle of magnifying the diameter of the coupled light by the use of lenses, bundles, or large diameter fiber.

However, the most common approach used here is the usage of lenses to extend the optical beam leading to minimizing the effects of mechanical misalignment.

Implementation: Single-pass FORJs are used primarily for digital data transmission and has been commercialized using the general design philosophy of multimode fiber. FORJs also follows the rule of directly combining the light across the gap without lenses. This is possible by the significantly greater core size of the Plastic Optical Fibre (approx.1.0 mm) which makes tolerances at all combining sites, including connectors, less critical. The success of Plastic Optical Fibre FORJ technology has its use in industrial applications, with the basic limitation being difficult to operate over shorter distances.

In such kinds, anti-reflective coatings are used on both the ends of the GRIN lenses to reduce reflections and remarkably improve the returned loss of the FORJ simultaneously improving the insertion loss to a lesser degree.

Without anti-reflective coatings, the reflectance at a glass

(refractive index, n_gl =1.5) to air (refractive index, n_air =1) interface normal to the beam path is
R = ((ngl – nair)/(ngl + nair))2 = 0.04
If all of this reflected light is coupled back into the input fiber the return loss (RL) is approximately 14 dB where
RL = -10 x log10 (R)

Multi-Pass FORJ

In these cases, multiple fibers enter the FORJ from either side of the rotating interface. In such kinds, the optical signals are coupled between special single selected pairs of fibers across the rotating interface enabling a multi-pass FORJ to spread multiple independent data streams all across the interface. This is common even when the optical multiplexing is not used.

A multi-pass FORJs are usually more complicated than single-pass FORJs because the specifics are required to provide rotary alignment of multiple fibers. Multi-pass FORJs are available in a variety of configurations and these kinds are usually physically larger than single-pass FORJs because of the existence of an increased number of fibers pairs and its complex structure.

Mirror-based Cell Approach

The mirror-based cell design is another way and has found its place in FORJ applications. This design is specifically designed to be useful in the marine environment where it is frequently required to fluid fill the FORJ in order to compensate for the sealing pressure.

In such configuration, a number of optical fibers enter the FORJ on one side of the rotating interface and, light from one fiber is reflected towards the rotation axis along with a short distance by using mirrors that are attached to a rotor rotating at the same speed in the same direction as the direction of the variety of fibers that entered the FORJ.

Within each cell, there is a third mirror that is held stationary to the cell and maintains the function of reflecting the beam of light located on the rotational axis of the FORJ towards the fiber aligned at 90 degrees to the rotation axis.

The light from a fiber that is combined across the rotary interface in a cell away from the variety of optical fibers that entered the FORJ must pass through the rotors of all individual cells closer to the variety of fibers.

Another important law is that each cell must also match its rotation, speed, and direction which can be achieved by clubbing the rotors to each other and to the variety of optical fibers using a gear train.

The mirrors that are attached to each of the cells are held in place by technique’s which specifically allows the light from fibers clubbed across the rotary interface in cells far from the variety of fibers to pass unimpeded. The distance between the two corresponding lenses attached with each pass increases as the cell-attached with the pass is located further from the variety of fibers.

Here, the loss of the fiber due to insertion increases, as a result, leveling up to the increase in the lens to lens distance usually limiting the number of fibers that pass through to ten or less.

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